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From Gary Helmling <ghelml...@gmail.com>
Subject Re: [DISCUSS] Make AsyncFSWAL the default WAL in 2.0
Date Thu, 12 May 2016 01:50:08 GMT
>
>
> I was trying to avoid the below oft-repeated pattern at least for the case
> of critical developments:
>
> + New feature arrives after much work by developer, reviewers and testers
> accompanied by fanfare (blog, talks).
> + Developers and reviewers move on after getting it committed or it gets
> hacked into a deploy so it works in a frankenstein form
> + It sits in our code base across one or more releases marked as optional,
> 'experimental'
> + The 'experimental' bleamish discourages its exercise by users
> + The feature lags, rots
> + Or, the odd time, we go ahead and enable it as default in spite of the
> fact it was never tried when experimental.
>
> Distributed Log Replay sat in hbase across a few major versions. Only when
> the threat of our making an actual release with it on by default did it get
> serious attention where it was found flawed and is now being actively
> purged. This was after it made it past reviews, multiple attempts at
> testing at scale, and so on; i.e. we'd done it all by the book. The time in
> an 'experimental' state added nothing.
>
>
Those are all valid concerns as well. It's certainly a pattern that we've
seen repeated. That's also a broader concern I have about the farther we
push out 2.0, then the less exercised master is.

I don't really know how best to balance this with concerns about user
stability.  Enabling by default in master would certainly be a forcing
function and would help it get more testing before release.  I hear that
argument.  But I'm worried about the impact after release, where something
as simple as a bug-fix point release upgrade of Hadoop could result in
runtime breakage of an HBase install.  Will this happen in practice?  I
don't know.  It seems unlikely that the private variable names being used
for example would change in a point release.  But we're violating the
abstraction that Hadoop provides us which guarantees such breakage won't
occur.


> > Yes. 2.0 is a bit out there so we have some time to iron out issues is
> the
> thought. Yes, it could push out delivery of 2.0.
>
>
Having this on by default in an unreleased master doesn't actually worry me
that much.  It's just the question of what happens when we do release.  At
that point, this discussion will be ancient history and I don't think we'll
give any renewed consideration to what the impact of this change might be.
Ideally it would be great to see this work in HDFS by that point and for
that HDFS version this becomes a non-issue.


>
> I think the discussion here has been helpful. Holes have been found (and
> plugged), the risk involved has gotten a good airing out here on dev, and
> in spite of the back and forth, one of our experts in good standing is
> still against it being on by default.
>
> If you are not down w/ the arguments, I'd be fine not making it the
> default.
> St.Ack
>

I don't think it's right to block this by myself, since I'm clearly in the
minority.  Since others clearly support this change, have at it.

But let me pose an alternate question: what if HDFS flat out refuses to
adopt this change?  What are our options then with this already shipping as
a default?  Would we continue to endure breakage due to the use of HDFS
private internals?  Do we switch the default back?  Do we do something else?

Thanks for the discussion.

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